The present invention relates, in general, to surface emitting and receiving photonic devices, and more particularly to improved surface emitting laser devices and methods for fabricating them.
Semiconductor lasers typically are fabricated by growing the appropriate layered semiconductor material on a substrate through Metalorganic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxy (MBE) to form an active layer parallel to the substrate surface. The material is then processed with a variety of semiconductor processing tools to produce a laser optical cavity incorporating the active layer, and metallic contacts are attached to the semiconductor material. Finally, laser mirror facets typically are formed at the ends of the laser cavity by cleaving the semiconductor material to define edges or ends of the laser optical cavity so that when a bias voltage is applied across the contacts the resulting current flow through the active layer causes photons to be emitted out of the faceted edges of the active layer in a direction perpendicular to the current flow.
The prior art also discloses processes for forming the mirror facets of semiconductor lasers through etching, allowing lasers to be monolithically integrated with other photonic devices on the same substrate. The formation of total-internal-reflection facets within an optical cavity through the creation of such facets at angles greater than the critical angle for light propagating within the cavity is also known.
The use of an etch process to form two total-internal-reflection facets at each end of a linear laser cavity, with each facet being positioned at an angle of 45° with respect to the plane of the active layer, is also described in the prior art. In such devices, light in the cavity may be directed perpendicularly upward at one end of the cavity, resulting in surface emission at one facet, while the facet at the other end of the cavity may be oppositely angled to direct the light perpendicularly downward to, for example, a high reflectivity stack below the laser structure.
The prior art also describes devices which combine etched 45° facets with cleaved facets. The resultant devices cannot be tested in a full-wafer and as such suffer from the same deficiencies as cleaved facet devices. Furthermore, they are incompatible with monolithic integration in view of the need for cleaving. Chao, et al., IEEE Photonics Technology Letters, volume 7, pages 836-838, attempted to overcome these short-comings, however, by providing an interrupted waveguide structure, but the resultant device suffered from scatter at each end of the laser cavity.
Vertical Cavity Surface Emitting Lasers (VCSELs), have gained popularity over the past several years; however, VCSELs do not allow in-plane monolithic integration of multiple devices and only allow light to exit their surface mirror at perpendicular incidence. A common aspect of these prior surface-emitting devices is that the photons are always emitted from the optical cavity in a direction perpendicular to the plane of the active layers.
A laser with low to no ripples in its far-field intensity profile is very desirable, for example, in efficient coupling of the laser beam into an optical fiber.